Isomerization processes are widely used by many refiners to rearrange the molecular structure of straight chain paraffinic hydrocarbons to more highly branched hydrocarbons. Generally, these more highly branched hydrocarbons have relatively high octane ratings.
Paraffin feeds for C5/C6 hydrocarbons isomerization processes can contain cyclic C6+ hydrocarbons such as benzene and cyclic C7+ hydrocarbons. As used herein, Cx hydrocarbons means hydrocarbon molecules that have “X” number of carbon atoms, Cx+ hydrocarbons means hydrocarbon molecules that have “X” and more than “X” number of carbon atoms, and Cx− hydrocarbons means hydrocarbon molecules that have “X” and less than “X” number of carbon atoms. As used herein, C5/C6 hydrocarbons mean C5 hydrocarbons and/or C6 hydrocarbons. When the weight percent of cyclic C6+ hydrocarbons in the paraffin feed is about 20% or greater (also referred to as an X-factor of 20 or greater in which the X-factor is defined as the sum of the weight percent of the cyclic C6 hydrocarbons plus C7+ hydrocarbons in the feed, e.g., benzene+cyclohexane+methylcyclopentane+C7+), the paraffin feed is considered a “hard feed to isomerize.” In particular, the C7+ hydrocarbons and to a much lesser extent C5 and C6 hydrocarbons in the reactor tend to hydrocrack, producing C4 hydrocarbons as well as other lighter hydrocarbons, e.g., C3− hydrocarbons. The C4 hydrocarbons have significant caloric and economic value. Unfortunately, the C4 hydrocarbons are traditionally removed in an off-gas stream of C5/C6 hydrocarbons isomerization processes and burned because the investment and operating costs for recovering the C4 hydrocarbons are prohibitively expensive.
Accordingly, it is desirable to provide methods and apparatuses for isomerization of paraffins including recovery of C4 hydrocarbons with reduced overall investment and/or operating costs. Moreover, it is desirable to provide methods and apparatuses for isomerization of paraffins including recovery of C4 hydrocarbons, such as for C5/C6 hydrocarbons isomerization processes that have paraffinic feed streams that may contain some C7+ hydrocarbons. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.